It is known to provide dual counterrotating propellers as the water-engaging drive element on a boat. The two propellers are of opposite hand but normally otherwise identical and are rotatable about the same axis immediately adjacent each other at the stern of the boat. Using two propellers reduces vibration, increases thrust, and eliminates reverse torque effective from the drive on the craft.
As described in U.S. Pat. No. 3,087,553 of Kostyun the rear propeller is fixed on the drive shaft and the front propellor is journaled on a fixed support tube coaxial with the rear shaft. Bevel gears rotatable about radial axes mesh with gear rings on the back and front edges of the front and rear propellers, respectively, so that the rotation of the front propeller is identical to but perfectly opposite that of the rear propeller.
Such an arrangement is fairly complex. The intermediate drive gears are fully exposed and liable to fouling so they wear rapidly. Furthermore retrofitting the system on an existing drive intended for use with a single propeller is difficult, particularly when the exhaust is discharged through the drive shaft, as is common.
Similarly it is standard to mount such counterrotating dual-propeller drives on torpedoes, principally because a single prop would rotate such a small submarine craft about its own axis. As described in U.S. Pat. No. 2,765,040 of Darrah, however, these arrangements are complex in the extreme and absolutely unfit for retrofitting on any standard drive.
The use of counterrotating propellers in aircraft is also well known, both in a helicopter as described in U.S. Pat. No. 3,933,324 of Ostrowski as well as in airplanes as described in U.S. Pat. Nos. 2,305,454, 2,322,394, and 2,406,460 respectively of Nallinger, Sharpe, and Guerke. All such systems are wholly unsuitable for watercraft.
In general the systems for driving counterrotating coaxial drive elements are quite complex. U.S. Pat. Nos. 2,543,453 and 3,646,834 respectively of Fuller and Davis evidence this complexity. As a result counterrotating dual propellers are virtually unknown on standard small pleasure craft, and have never been meaningfully applied to small outboard-motor drives at all.